U.S. patent application number 12/349145 was filed with the patent office on 2009-07-23 for lateral electric field type liquid crystal display device.
This patent application is currently assigned to NEC LCD TECHNOLOGIES, LTD.. Invention is credited to Hideki Itou, Shinichi Nishida, Yoshikazu Sakaguchi, Teruaki Suzuki, Sounosuke Takahashi.
Application Number | 20090185090 12/349145 |
Document ID | / |
Family ID | 40876186 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090185090 |
Kind Code |
A1 |
Suzuki; Teruaki ; et
al. |
July 23, 2009 |
LATERAL ELECTRIC FIELD TYPE LIQUID CRYSTAL DISPLAY DEVICE
Abstract
A lateral electric field type LCD device makes it possible to
increase the degree of freedom in designing the constituent
elements thereof and to improve the aperture ratio easily compared
with the related-art LCD structure. The drain bus lines are
entirely covered with the at least one first liquid crystal driving
electrode (e.g., the common electrode). The gate bus line
corresponding to each pixel region is covered with the at least one
first liquid crystal driving electrode except for a predetermined
non-overlapped area existing in a part that does not overlap witt
the corresponding TFT. The predetermined non-overlapped area of the
gate bus line corresponding to each pixel region is covered with
the storage capacitor electrode corresponding to the adjacent pixel
region. Preferably, the at least one first liquid crystal driving
electrode comprises openings that expose the channel regions of the
TFTs, respectively.
Inventors: |
Suzuki; Teruaki; (Kanagawa,
JP) ; Itou; Hideki; (Kanagawa, JP) ;
Takahashi; Sounosuke; (Kanagawa, JP) ; Nishida;
Shinichi; (Kanagawa, JP) ; Sakaguchi; Yoshikazu;
(Kanagawa, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
209 Madison Street, Suite 500
ALEXANDRIA
VA
22314
US
|
Assignee: |
NEC LCD TECHNOLOGIES, LTD.
Kanagawa
JP
|
Family ID: |
40876186 |
Appl. No.: |
12/349145 |
Filed: |
January 6, 2009 |
Current U.S.
Class: |
349/39 |
Current CPC
Class: |
G02F 2201/123 20130101;
G02F 1/134363 20130101; G02F 2201/124 20130101 |
Class at
Publication: |
349/39 |
International
Class: |
G02F 1/1343 20060101
G02F001/1343 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 18, 2008 |
JP |
2008-009919 |
Nov 7, 2008 |
JP |
2008-287219 |
Claims
1. A lateral electric field type liquid crystal display device
comprising: a first substrate and a second substrate placed
opposite to each other at an approximately constant gap; a liquid
crystal layer formed between the first substrate and the second
substrate; drain bus lines formed on the first substrate; gate bus
lines formed on the first substrate in such a way as to be
intersected with the drain bus lines; pixel regions defined in a
matrix array by the drain bus lines and the gate bus lines; at
least one first liquid crystal driving electrode and second liquid
crystal driving electrodes formed on the first substrate; thin-film
transistors formed on the first substrate for the respective pixel
regions; and storage capacitor electrodes formed on the first
substrate for the respective pixel regions; wherein an alignment
direction of liquid crystal molecules existing in the liquid
crystal layer is rotated in planes approximately parallel to the
first substrate and the second substrate by applying liquid crystal
driving electric field to the liquid crystal layer using the at
least one first liquid crystal driving electrode and the second
liquid crystal driving electrodes, thereby displaying images; the
drain bus lines are entirely covered with the at least one first
liquid crystal driving electrode; the gate bus line corresponding
to each of the pixel regions is covered with the at least one first
liquid crystal driving electrode except for a predetermined
non-overlapped area existing in a part that does not overlap with
the corresponding thin-film transistor; and the predetermined
non-overlapped area of the gate bus line corresponding to each of
the pixel regions is covered with the storage capacitor electrode
corresponding to the adjacent pixel region.
2. The liquid crystal display device according to claim 1, wherein
the gate bus line corresponding to each of the pixel regions is
placed in a lower layer than the corresponding storage capacitor
electrode; and the at least one first liquid crystal driving
electrode is placed in an upper layer than the corresponding
storage capacitor electrode; and wherein in a vicinity of each of
the non-overlapped areas, the storage capacitor electrode is
overlapped with the adjacent gate bus line in such a way as to
exceed across a first side edge of the same gate bus line, and the
at least one first liquid crystal driving electrode is overlapped
with the same gate bus line in such a way as to exceed across a
second side edge thereof opposite to the first side edge.
3. The liquid crystal display device according to claim 2, wherein
the at least one first liquid crystal driving electrode is
partially overlapped with the corresponding storage capacitor
electrode in each of the non-overlapped areas.
4. The liquid crystal display device according to claim 2, wherein
the at least one first liquid crystal driving electrode does not
exceed across the first side edge of the gate bus line in each of
the non-overlapped areas.
5. The liquid crystal display device according to claim 1, wherein
the at least one first liquid crystal driving electrode comprises
openings formed in such a way as to expose channel regions of the
thin-film transistors, respectively.
6. The liquid crystal display device according to claim 5, wherein
opposite edges formed by each of the openings of the at least one
first liquid crystal driving electrode have a smaller width than
the corresponding gate bus line.
7. The liquid crystal display device according to claim 1, further
comprising light-shielding regions formed on the second substrate
at positions opposite to channel regions of the thin-film
transistors; wherein each of the light-shielding regions has an
isolated pattern; and the light-shielding regions are formed
corresponding to the respective pixel regions.
8. The liquid crystal display device according to claim 7, wherein
the light-shielding regions are formed by overlapping at least two
of color layers that constitute a color filter.
9. The liquid crystal display device according to claim 7, wherein
the light-shielding regions have an optical density (OD) value that
is equal to 1.5 or greater and that is equal to 3.0 or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a Liquid Crystal Display
(LCD) device and more particularly, to an active-matrix addressing
LCD device of the lateral electric field type, such as the In-Plane
Switching (IPS) type. The present invention is applicable to
monitors designed for computers using the lateral electric field
type LCD device, LCD televisions, portable telephone terminals,
Global Positioning System (GPS) terminals, car navigation systems,
video game machines, Automatic Teller Machine (ATM) terminals
located at banks or convenience stores, medical diagnosis
apparatuses, and so on.
[0003] 2. Description of the Related Art
[0004] Generally, the LCD device has the features such as low
profile, reduced weight, and low power consumption. In particular,
the active-matrix addressing LCD device that drives the respective
pixels arranged vertically and horizontally in a matrix array by
the active elements has ever been recognized as a high image
quality flat-panel display device. Especially, the active-matrix
addressing LCD device using thin-film transistors (TFTs) as the
active elements for switching the respective pixels has been
extensively diffused.
[0005] Most of active-matrix addressing LCD devices, which utilizes
the electrooptic effects of the TN (Twisted Nematic) type liquid
crystal material sandwiched by two substrates, display images by
the application of an electric field approximately vertical to the
main surfaces of the substrates across the liquid crystal material
to thereby cause displacement of the liquid crystal molecules of
the said material. These LCD devices are termed the "vertical
electric field" type. On the other hand, some of the active-matrix
addressing LCD devices display images by the application of an
electric field approximately parallel to the main surfaces of the
substrates across the liquid crystal material to thereby cause
displacement of the liquid crystal molecules of the said material
in the planes parallel to the said main surfaces. These LCD devices
have been known also, which are termed the "lateral electric field"
type. Various improvements have ever been made not only for the
vertical electric field type LCD devices but also for the lateral
electric field type ones. Some of the improvements made for the
latter will be exemplified below.
[0006] For example, the Patent Document 1 (Japanese Unexamined
Patent Publication No. 2000-089240) published on Mar. 31, 2000 and
the Patent Document 2 Japanese Unexamined Patent Publication No.
2004-062145) published on Feb. 26, 2004 disclose lateral electric
field type LCD devices, each of which comprises drain bus lines and
gate bus lines covered with a common electrode or electrodes in
such a way that an interlayer insulating film intervenes between
the drain and gate bus lines and the common electrode(s). The
structure of the LCD device disclosed in the Patent Document 2 is
shown in FIGS. 1, 2A to 2C, and 3.
[0007] FIG. 1 is a plan view showing the structure of the
active-matrix substrate (i.e., the TFT substrate) of the said LCD
device, FIGS. 2A, 2B and 2C are plan views showing the structures
of the three layers that constitute the said active-matrix
substrate, respectively, and FIG. 3 is an enlarged partial plan
view showing the detailed structure of the vicinity of the gate bus
line of the said active-matrix substrate. Since all the pixels of
the active-matrix addressing LCD device have the same structure,
the structure of one pixel is shown in FIGS. 1 to 3.
[0008] As clearly shown in FIGS. 2A, 2B and 2C, the active-matrix
substrate of the related-art LCD device shown in FIG. 1 comprises
gate bus lines 155 and common bus lines 152 formed in the same
layer on a transparent insulative plate (e.g., a glass plate)(not
shown); drain bus lines 156, pixel electrodes 171, TFTs 145, and
storage capacitor electrodes 173 formed in the same layer on a gate
insulating film (not shown) that covers the gate and common bus
lines 155 and 152; and a common electrode 172 formed on a
protective insulating film (not shown) that covers the drain bus
lines 156, the pixel electrodes 171, the TFTs 145, and the storage
capacitor electrodes 173. It is usual that the pixel electrodes 171
and the common electrode 172 are respectively formed by patterning
transparent conductive metal films made of, for example, Indium Tin
Oxide (ITO).
[0009] The gate bus lines 155 extending in parallel to each other
at equal intervals along the lateral (horizontal) direction of FIG.
1 and the drain bus lines 156 extending in parallel to each other
at equal intervals along the longitudinal (vertical) direction of
the same figure define rectangular regions. Each of these
rectangular regions forms a pixel region. These pixel regions
(i.e., the pixels) are arranged in a matrix array as a whole. Each
of the TFTs 145 is located near one of the intersections formed by
the two gate bus lines 155 and the two drain bus lines 156 that
define each pixel region (i.e., at the lower left intersection in
FIG. 1). Similar to the gate bus lines 155, the common bus lines
152 extend along the lateral direction of the same figure in
parallel with the gate bus lines 155. Each of the common bus lines
152 is located at the opposite side to the TFT 145 (i.e., at the
upper end in FIG. 1) in the pixel region. In other words, it is
placed near one of the two gate bus lines 155 that is located on
the distant side from the TFT 145 in the pixel region, (i.e., the
gate bus line 155 at the upper position in FIG. 1). Therefore, it
may be said that each of the common bus lines 152 is located near
the TFTs 145 existing in the preceding pixel regions that are
upwardly adjacent thereto along the extension direction of the
drain bus lines 156 (i.e., the vertical direction) to be apart from
the said TFTs 145.
[0010] The drain electrode 144, the source electrode 142, and the
semiconductor film 143 of the TFT 145 are respectively formed to
have such patterns or shapes as shown in FIG. 2B. The gate
electrode (not shown) of the TFT 145 is formed to be united with
the gate bus line 155, in other words, the gate electrode is a part
of the gate bus line 155. The gate electrode is placed at a
position overlapping with the semiconductor film 143 between the
drain electrode 144 and the source electrode 142. It is usual that
an amorphous silicon film is used as the semiconductor film
143.
[0011] The pixel electrode 171 and the common electrode 172, which
are provided for generating liquid crystal driving electric field,
are formed to have such patterns or shapes as shown in FIGS. 2B and
2C, respectively. Each pixel electrode 171 and the common electrode
172 comprise comb-tooth like parts (i.e., thin belt-shaped parts
protruding into the pixel region) 171a and 172a that are mated with
each other, respectively. Here, the total number of the comb-tooth
like parts 171a of the pixel electrode 171 is three; on the other
hand, the total number of the comb-tooth like parts 172a of the
common electrode 172 in each pixel region is two. The common
electrode 172 further comprises openings or windows 172b formed
respectively at the positions overlapped with the channel regions
of the TFTs 145. For this reason, the whole channel region of the
TFT 145 is exposed from the opening 172b in such a way as not to
overlap with the common electrode 172. This is to avoid the change
of the characteristics of the TFT 145 caused by the back gate
effect.
[0012] The base of the pixel electrode 171, which is located on the
side of the source electrode 142, is connected mechanically and
electrically to the source electrode 142 of the TFT 145. Moreover,
the ends of the three comb-tooth like parts 171a of the pixel
electrode 171, which are located on the opposite side to the source
electrode 142 in the pixel region, are connected mechanically and
electrically to the storage capacitor electrode 173. The common
electrode 172, which is commonly used for all the pixel regions, is
connected electrically to the underlying common bus lines 152 by
way of the corresponding contact holes 162 penetrating through the
gate insulating film and the protective insulating film in the
respective pixel regions.
[0013] The storage capacitor electrode 173 is placed at a position
overlapped with the common bus line 152 that is directly under the
electrode 173 in each pixel region, where the gate insulating film
intervenes between the storage capacitor electrode 173 and the
common bus line 152. The storage capacitor is formed by the
overlapped parts of the storage capacitor electrode 173 and the
corresponding common bus line 152. In other words, the storage
capacitor is constituted by the storage capacitor electrode 173,
the corresponding common line 152, and the gate insulting film
intervening between them. As shown in FIG. 3, the storage capacitor
is not overlapped with the gate bus line 155 that is adjacent to
the corresponding common bus line 152.
[0014] As clearly seen from FIGS. 2B, 2C and 3, the common
electrode 172 covers the entirety of the drain bus lines 156
extending along the vertical direction of the same figures and the
entirety of the gate bus lines 155 extending along the lateral
direction of the same figures (except for the openings 172b).
Moreover, the common electrode 172 is formed to cover not only the
areas directly above the gate bus lines 155 but also the gaps
between the gate bus lines 155 and the common bus lines 152
adjacent thereto (each of the adjacent common bus lines 152 is
located in the subsequent pixel regions that are downwardly
adjacent thereto along the extension direction of the drain bus
lines 156, i.e., the vertical direction), the gaps between the gate
bus lines 155 and the corresponding source electrodes 142, the gaps
between the gate bus lines 155 and the adjacent storage capacitor
electrodes 173, and the peripheral areas of the edges of the source
electrodes 142 and the adjacent storage capacitor electrodes 173.
For this reason, the electric field generated near the gate bus
lines 155 can be shielded by the common electrode 172. As seen from
FIG. 3, the edges 172c of the common electrode 172 located on the
side of the storage capacitor electrodes 173 (which are
respectively extended along the adjacent gate bus lines 155) are
not overlapped with the gate bus lines 155.
[0015] The reference numeral 181 shown in FIG. 3 denotes the black
matrix layer formed on the opposite substrate. The black matrix
layer 181 comprises rectangular light-shielding regions provided
for the respective pixel regions. Each of the light-shielding
regions is defined by a rectangular broken line in FIG. 3. Each of
the light--shielding regions has a size that covers the whole TFT
145 and is isolated to have a rectangular island-like shape. In
this way, the occupation area of each light-shielding region of the
black matrix layer 181 is restricted to a minimum necessary for
preventing the entry of light into the TFT 145. The prevention of
the entry of light into (the channel region of) the TFT 145 by the
light-shielding region is to prevent the functions of the TFT 145
from being hindered due to the incident light.
[0016] With the active matrix substrate of the related-art LCD
device shown in FIGS. 1 to 3, as explained above, the electric
field generated in the vicinities of the gate bus lines 155 can be
shielded by the common electrode 172 placed in an upper layer than
the gate bus lines 155. Therefore, the alignment direction of the
liquid crystal molecules existing in the peripheral areas of the
gate bus lines 155 is not changed from their initial alignment
direction, which means that optical leakage does not occur in the
same peripheral areas. Accordingly, it is unnecessary to shield the
light in the same peripheral areas on the opposite substrate, and
the size of each light-shielding region can be restricted to a
minimum, as shown in FIG. 3.
[0017] On the other hand, with the active matrix substrate of the
related-art LCD device shown in FIGS. 1 to 3, the pixel electrodes
171 may be made of the same transparent conductive metal as the
common electrode 172. The structure of the active matrix substrate
in this case will be explained below with reference to FIGS. 4 to
10.
[0018] FIG. 4 is a plan view showing the structure of the
active-matrix substrate of the LCD device having the structure that
the pixel electrodes 171 are made of the same transparent
conductive metal as the common electrode 172. FIGS. 5A, 5B and 5C
are plan views showing the structures of the three layers that
constitute the said active-matrix substrates respectively. FIG. 6
is an enlarged partial plan view showing the detailed structure of
the vicinity of the gate bus line of the said active-matrix
substrate. FIG. 7 is a partial cross-sectional view of the said LCD
device along the line VII-VII in FIG. 6. FIGS. 8A and 8B are
partial cross-sectional views of the said LCD device along the
lines VIIIA-VIIIA and VIIIB-VIIIB in FIG. 6, respectively. FIG. 9
is a plan view obtained by omitting the pixel electrode 171 and the
common electrode 172 in FIG. 4 for facilitating the understanding
of the understructures of them. FIG. 10 is an enlarged partial plan
view obtained by omitting the pixel electrode 171, the common
electrode 172, the black matrix layer 181, and the contact holes
161 and 162 in FIG. 6 for facilitating the understanding of the
understructures of them. These figures also show the structure of
one pixel.
[0019] As seen from FIGS. 5A, 5B and 5C, the structure of FIGS. 4
to 10 has the following differences from that of FIGS. 1 to 3: (a)
The pixel electrodes 171 are made of the same transparent
conductive metal as the common electrode 172 and are placed in the
same layer as the common electrode 172.
[0020] (b) Auxiliary pixel electrodes 170 are formed on the same
layer as the drain bus lines 156.
[0021] (c) The pixel electrodes 171 are connected electrically to
the corresponding storage capacitor electrodes 173 placed in the
lower layer than the pixel electrodes 171 by way of the
corresponding contact holes 161 penetrating through the protective
insulating film 159 (see FIG. 7 and FIGS. 8A and 8B), and
furthermore, the pixel electrodes 171 are connected electrically to
the corresponding source electrodes 142 by way of the corresponding
auxiliary pixel electrodes 170.
[0022] The active-matrix substrate structure shown in FIGS. 4 to 10
is the same as that of FIGS. 1 to 3 except for the above-described
differences (a) to (c). Therefore, explanation about the same
structural elements as those of the active-matrix substrate
structure of the related-art LCD device explained with reference to
FIGS. 1 to 3 is omitted here by attaching the same reference
numerals as used in FIGS. 1 to 3 to the same structural
elements.
[0023] The pixel electrode 171 and the common electrode 172 are
respectively formed to have such patterns or shapes as shown in
FIG. 5C. Each pixel electrode 171 and the common electrode 172
comprise respectively comb-tooth like parts (i.e., thin belt-shaped
parts protruding into the pixel region) 171a and 172a that are
mated with each other in the state shown in the same figure. Here,
the total number of the comb-tooth like parts 171a of the pixel
electrode 171 is three; on the other hand, the total number of the
comb-tooth like parts 172a of the common electrode 172 in each
pixel region is two.
[0024] The auxiliary pixel electrodes 170 are formed in the same
layer as the drain bus lines 156. The shape of each auxiliary pixel
electrode 170 corresponds to the shape formed by the combination of
the base of the pixel electrode 171 in the structure shown in FIGS.
1 to 3 and the central comb-tooth like part 171a thereof. The
bottom end of the auxiliary pixel electrodes 170 is connected
mechanically and electrically to the source electrode 142 and the
top end thereof is connected mechanically and electrically to the
storage capacitor electrode 173. In this way, the pixel electrode
171 is electrically connected to the source electrode 142 by way of
the storage capacitor electrode 173 and the auxiliary pixel
electrode 170 in the pixel region.
[0025] As shown in FIGS. 9 and 10, the storage capacitor electrode
173 is overlapped with the corresponding common bus line 152
located right below the said storage capacitor electrode 173;
however, the storage capacitor electrode 173 is not overlapped with
the adjacent gate bus line 155. The common electrode 172 covers the
entirety of the corresponding gate bus line 155 and therefore, the
edge 172c of the common electrode 172 (which is extended along the
adjacent gate bus line 155) located on the side of the storage
capacitor electrode 173 is not overlapped with the adjacent gate
bus line 155. This point is the same as the structure of FIGS. 1 to
3.
[0026] Next, the entire configuration of the related-art LCD device
shown in FIGS. 4 to 10 will be explained below with reference to
FIG. 7 and FIGS. 8A and 8B.
[0027] This LCD device is configured by coupling and uniting the
active-matrix substrate and the opposite substrate with each other
in such a way that a liquid crystal layer 120 intervenes between
these two substrates.
[0028] The active-matrix substrate comprises a transparent glass
plate 111; and the common bus lines 152, the gate bus lines 155,
the drain bus lines 156, the TFTs 145, the auxiliary pixel
electrodes 170, the pixel electrodes 171, the common electrode 172,
and the storage capacitor electrodes 173 formed on or over the
inner surface of the glass plate 111. The common bus lines 152 and
the gate bus lines 155, which are placed directly on the inner
surface of the glass plate 111, are covered with the gate
insulating film 157 except for the positions corresponding to the
contact holes 162. The drain electrodes 144, the source electrodes
142, and the semiconductor films 143 of the TFTs 145; the auxiliary
pixel electrodes 170; the storage capacitor electrodes 173; and the
drain bus lines 156 are placed on the gate insulating film 157.
Therefore, the common bus lines 152 and the gate bus lines 155 are
electrically insulated from the drain electrodes 144, the source
electrodes 142, and the semiconductor films 143, the auxiliary
pixel electrodes 170, the storage capacitor electrodes 173, and the
drain bus lines 156 by the gate insulating film 157. These
structures formed on the glass plate 111 are covered with the
protective insulating film 159 except for the positions
corresponding to the contact holes 161 and 162.
[0029] The pixel electrodes 171 and the common electrode 172 are
placed on the protective insulating film 159. As explained above,
the pixel electrode 171 is electrically connected to the
corresponding storage capacitor electrode 173 located right under
the same pixel electrode 171 by way of the corresponding contact
hole 161 (which penetrates through the protective insulating film
159) and to the corresponding source electrode 142 by way of the
corresponding auxiliary pixel electrode 170 in the pixel region.
The common electrode 172 is electrically connected to the common
bus lines 152 located right under the common electrode 172 by way
of the corresponding contact holes 162 (which penetrates through
the protective insulating film 159 and the gate insulating film
157) in the respective pixel regions. The pixel electrodes 171 and
the common electrode 172 are respectively formed by pattering
transparent conductive metal films, for example, ITO films.
[0030] The surface of the active-matrix substrate having the
above-described structure (i.e., the surface on which the pixel
electrodes 171 and the common electrode 172 are formed) is covered
with an alignment film 131 made of an organic polymer. The surface
of the alignment film 131 has been subjected to a predetermined
aligning treatment for aligning the initial alignment direction of
the liquid crystal molecules existing in the liquid crystal layer
120 to a desired direction.
[0031] On the other hand, the opposite substrate (which may be
termed the color filter substrate) comprises a transparent glass
plate 112; a color filter (not shown) formed by three color layers
182R, 182G, and 182B of the three primary colors, i.e., red (R),
green (G) and blue (B), formed on the inner surface of the glass
plate 112 corresponding to the arrangement of the respective pixel
regions; and the black matrix layer 181 for optical shielding.
Similar to the structure of FIGS. 1 to 3, the black matrix layer
181 comprises the rectangular light-shielding regions for the
respective pixel regions, each of which is defined by the broken
line in FIG. 6. In addition, the three color layers 182R, 182G, and
182B are generically termed the color layer 182.
[0032] The color layer (i.e., the color filter) 182 and the black
matrix layer 181 are covered with an overcoat layer 185 made of an
acrylic resin. Columnar spacers (not shown) are formed on the inner
surface of the overcoat layer 185 to keep the gap between the
active-matrix substrate and the opposite substrate. The inner
surface of the overcoat layer 185 is covered with an alignment film
132 made of an organic polymer. The surface of the alignment film
132 has been subjected to a predetermined aligning treatment for
aligning the initial alignment direction of the liquid crystal
molecules existing in the liquid crystal layer 120 to a desired
direction.
[0033] The active-matrix substrate and the opposite substrate each
having the above-described structure are superposed on each other
at a predetermined gap in such a way that their surfaces on which
the alignment films 131 and 132 are respectively formed are
directed inwardly and opposed to each other. The liquid crystal
layer 120 is formed in the gap between the active-matrix and
opposite substrates. To confine the liquid crystal material
existing in the liquid crystal layer 120 into the gap between the
two substrates, the outer edges of the two substrates are sealed
with a sealing material (not shown). A pair of polarizer plates
(not shown) is arranged on the outer surfaces of the two
substrates, respectively.
[0034] In addition, the Patent Document 3 (Japanese Unexamined
Patent Publication No. 2000-029014) published on Jan. 21, 2000 and
the Patent Document 4 (Japanese Unexamined Patent Publication No.
2002-082630) published on Mar. 22, 2002 disclose the technique for
forming the light-shielding regions of the black matrix layer 181
by overlapping the end portions of the adjoining color layers of
the color filter, where the black matrix layer 181 is not used. If
this technique is employed, the formation processes of the black
matrix layer 181 can be omitted and as a result, the fabrication
cost lowering is realizable.
[0035] With the above-described two structures of the related-art
LCD devices, the whole surfaces of the respective gate bus lines
155 are covered with the common electrode 172 placed in the upper
layer than the gate bus lines 155. This is to prevent the optical
leakage caused by the alignment direction change of the liquid
crystal molecules from their initial alignment direction in the
peripheral areas of the gate bus lines 155 and the TFTs 145 due to
the electric field generated in the same peripheral areas. However,
in the case where the whole surfaces of the respective gate bus
lines 155 are covered with the common electrode 172 in this way,
there is a problem that the degree of freedom in designing the
pattern and layout of the respective constituent elements of these
two related-art LCD devices is low and as a result, it is difficult
to improve the aperture ratio.
[0036] As another method of preventing such the optical leakage as
explained above, a method of broadening the respective
light-shielding regions of the black matrix layer 181 that are
arranged on the opposite substrate at the predetermined positions
overlapped with the respective gate bus lines 155 is known. In this
method, however, it is necessary to broaden the respective
light-shielding regions sufficiently in consideration of the
margins for the positional deviation occurring in the coupling
operation of the active matrix substrate and the opposite (or color
filter) substrate. Accordingly, in this case also, it is difficult
to realize a high aperture ratio.
[0037] In the case where the light-shielding regions are formed by
overlapping the end portions of the different color layers of the
color filter instead of the use of the black matrix layer 181, as
disclosed in the Patent Documents 3 and 4, fabrication cost
lowering may be realized due to the omission of the formation
processes of the black matrix layer 181. In this case also,
however, it is necessary to form the sufficiently wide
light-shielding regions on the opposite substrate in consideration
of the margins for the positional deviation between the
active-matrix substrate and the opposite substrate. Therefore, it
is difficult to realize a high aperture ratio.
[0038] Furthermore, there is another problem that the large level
difference formed by overlapping the end portions of the different
color layers affects badly the alignment of the liquid crystal
molecules and/or prolong the time required for the injection
process of the liquid crystal material.
SUMMARY OF THE INVENTION
[0039] The present invention was created in consideration of the
above-described problems or disadvantages.
[0040] An object of the present invention is to provide a lateral
electric field type LCD device that makes it possible to increase
the degree of freedom in designing the constituent elements thereof
and to improve the aperture ratio easily compared with the
related-art LCD structure shown in FIGS. 4 to 10.
[0041] Another object of the present invention is to provide a
lateral electric field type LCD device that makes it possible to
increase the luminance or to reduce the electric power consumption
compared with the related-art LCD structure shown in FIGS. 4 to
10.
[0042] The above objects together with others not specifically
mentioned will become clear to those skilled in the art from the
following description.
[0043] A lateral electric field type liquid crystal display device
according to the present invention comprises:
[0044] a first substrate and a second substrate placed opposite to
each other at an approximately constant gap;
[0045] a liquid crystal layer formed between the first substrate
and the second substrate;
[0046] drain bus lines formed on the first substrate;
[0047] gate bus lines formed on the first substrate in such a way
as to be intersected with the drain bus lines;
[0048] pixel regions defined in a matrix array by the drain bus
lines and the gate bus lines;
[0049] at least one first liquid crystal driving electrode and
second liquid crystal driving electrodes formed on the first
substrate;
[0050] thin-film transistors formed on the first substrate for the
respective pixel regions; and
[0051] storage capacitor electrodes formed on the first substrate
for the respective pixel regions;
[0052] wherein an alignment direction of liquid crystal molecules
existing in the liquid crystal layer is rotated in planes
approximately parallel to the first substrate and the second
substrate by applying liquid crystal driving electric field to the
liquid crystal layer using the at least one first liquid crystal
driving electrode and the second liquid crystal driving electrodes,
thereby displaying images;
[0053] the drain bus lines are entirely covered with the at least
one first liquid crystal driving electrode;
[0054] the gate bus line corresponding to each of the pixel regions
is covered with the at least one first liquid crystal driving
electrode except for a predetermined non-overlapped area existing
in a part that does not overlap with the corresponding thin-film
transistor; and
[0055] the predetermined non-overlapped area of the gate bus line
corresponding to each of the pixel regions is covered with the
storage capacitor electrode corresponding to the adjacent pixel
region.
[0056] With the lateral electric field type liquid crystal display
device according to the present invention, the drain bus lines are
entirely covered with the at least one first liquid crystal driving
electrode (which corresponds to, for example, a common electrode),
and at the same time, the gate bus line corresponding to each of
the pixel regions is covered with the at least one first liquid
crystal driving electrode except for the predetermined
non-overlapped area existing in the part that does not overlap with
the corresponding thin-film transistor. Moreover, the predetermined
non-overlapped area of the gate bus line corresponding to each of
the pixel regions (i.e., the area of the gate bus line that is not
covered with the at least one first liquid crystal driving
electrode in the part that does not overlap with the corresponding
thin-film transistor) is covered with the storage capacitor
electrode corresponding to the adjacent pixel region to the said
gate bus line. Therefore, similar to the structure of the
related-art LCD device shown in FIGS. 4 to 10, the electric field
generated in the vicinities of the respective gate bus lines can be
shielded effectively by the at least one first liquid crystal
driving electrode.
[0057] As a result, it is unnecessary that the shape or pattern of
the at least one first liquid crystal driving electrode is
restricted to a shape or pattern that covers the whole surface of
the gate bus line corresponding to each pixel region (except for
the part that overlaps with the corresponding thin-film transistor)
as used in the above-described structure of the related-art LCD
device shown in FIGS. 4 to 10. This means that the at least one
first liquid crystal driving electrode may have a shape or pattern
that includes a part that does not cover the gate bus line
corresponding to each pixel region.
[0058] Accordingly, the restriction that the gate bus line
corresponding to each of the pixel regions is entirely covered with
the at least one first liquid crystal driving electrode in the part
that does not overlap with the corresponding thin-film transistor,
which is included in the above-described structure of the
related-art LCD device shown in FIGS. 4 to 10, is eliminated. In
this way, the degree of freedom in designing the constituent
elements of a LCD device of this type can be increased.
[0059] Moreover, because of the elimination of the above-described
restriction, the positions of the contact holes and those of the
ends of the second liquid crystal driving electrodes (which
correspond to, for example, pixel electrodes) can be shifted in due
order toward the outer edges of the pixel regions. Therefore, the
aperture ratio can be improved (in other words, a higher aperture
ratio can be realized) easily compared with the above-described
related-art LCD structure shown in FIGS. 4 to 10.
[0060] Due to the improvement of the aperture ratio, if the amount
of emitted light from the backlight unit is not changed, the
luminance can be increased compared with the related-art LCD
structure shown in FIGS. 4 to 10. Due to the same reason, if the
luminance is not changed, the electric power consumption can be
reduced compared with the related-art LCD structure shown in FIGS.
4 to 10.
[0061] In addition, by properly adjusting the shape and/or the
position of the storage capacitor electrode, a desired storage
capacitance can be ensured easily without lowering the aperture
ratio or while improving the aperture ratio.
[0062] In a preferred embodiment of the LCD device according to the
present invention, the gate bus line corresponding to each of the
pixel regions is placed in a lower layer than the corresponding
storage capacitor electrode, and the at least one first liquid
crystal driving electrode is placed in an upper layer than the
corresponding storage capacitor electrode. Moreover, in a vicinity
of each of the non-overlapped areas, the storage capacitor
electrode is overlapped with the adjacent gate bus line in such a
way as to exceed across a first side edge of the same gate bus
line, and the at least one first liquid crystal driving electrode
is overlapped with the same gate bus line in such a way as to
exceed across a second side edge thereof opposite to the first side
edge.
[0063] In this embodiment, it is preferred that the at least one
first liquid crystal driving electrode is partially overlapped with
the corresponding storage capacitor electrode in each of the
non-overlapped areas. In addition, it is preferred that the at
least one first liquid crystal driving electrode does not exceed
across the first side edge of the gate bus line in each of the
non-overlapped areas.
[0064] In another preferred embodiment of the LCD device according
to the present invention, the at least one first liquid crystal
driving electrode comprises openings formed in such a way as to
expose channel regions of the TFTs, respectively.
[0065] In this embodiment, it is preferred that opposite edges
formed by each of the openings of the at least one first liquid
crystal driving electrode have a smaller width than the
corresponding gate bus line.
[0066] In still another preferred embodiment of the LCD device
according to the present invention, light-shielding regions are
formed on the second substrate at positions opposite to channel
regions of the TFTs. Each of the light-shielding regions has an
isolated pattern. The light-shielding regions are formed
corresponding to the respective pixel regions.
[0067] In this embodiment, it is preferred that the light-shielding
regions are formed by overlapping at least two of color layers that
constitute a color filter. In addition, it is preferred that the
light-shielding regions have an optical density (OD) value that is
equal to 1.5 or greater and that is equal to 3.0 or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] In order that the present invention may be readily carried
into effect, it will now be described with reference to the
accompanying drawings.
[0069] FIG. 1 is a plan view showing the structure of the
active-matrix substrate of a related-art lateral electric field
type LCD device.
[0070] FIGS. 2A, 2B and 2C are plan views showing the structures of
the three layers that constitute the active-matrix substrate of the
related-art lateral electric field type LCD device of FIG. 1,
respectively.
[0071] FIG. 3 is an enlarged partial plan view showing the detailed
structure of the vicinity of the gate bus line of the related-art
lateral electric field type LCD device of FIG. 1.
[0072] FIG. 4 is a plan view showing the structure of the
active-matrix substrate obtained by forming the pixel electrodes
171 by the same transparent conductive metal as the common
electrode 172 in the related-art lateral electric field type LCD
device of FIG. 1.
[0073] FIGS. 5A, 5B and 5C are plan views showing the structures of
the three layers that constitute the active-matrix substrate of the
related-art lateral electric field type LCD device of FIG. 4,
respectively.
[0074] FIG. 6 is an enlarged partial plan view showing the detailed
structure of the vicinity of the gate bus line of the related-art
lateral electric field type LCD device of FIG. 4.
[0075] FIG. 7 is a partial cross-sectional view of the related-art
lateral electric field type LCD device of FIG. 4 along the line
VII-VII in FIG. 6.
[0076] FIGS. 8A and 8B are partial cross-sectional views of the
related-art lateral electric field type LCD device of FIG. 4 along
the lines VIIIA-VIIIA and VIIIB-VIIIB in FIG. 6, respectively.
[0077] FIG. 9 is a plan view obtained by omitting the pixel
electrode 171 and the common electrode 172 in FIG. 4 for
facilitating the understanding of the understructures thereof.
[0078] FIG. 10 is an enlarged partial plan view obtained by
omitting the pixel electrode 171, the common electrode 172, the
black matrix layer 181, and the contact holes 161 and 162 in FIG. 4
for facilitating the understanding of the understructures
thereof.
[0079] FIG. 11 is a plan view showing the structure of the
active-matrix substrate of a lateral electric field type LCD device
according to a first embodiment of the present invention.
[0080] FIGS. 12A, 12B and 12C are plan views showing the structures
of the three layers that constitute the active-matrix substrate of
the lateral electric field type LCD device according to the first
embodiment of FIG. 10, respectively.
[0081] FIG. 13 is an enlarged partial plan view showing the
detailed structure of the vicinity of the gate bus line of the
lateral electric field type LCD device according to the first
embodiment of FIG. 10.
[0082] FIG. 14 is a partial cross-sectional view of the lateral
electric field type LCD device according to the first embodiment of
FIG. 10 along the line XIV-XIV in FIG. 13.
[0083] FIGS. 15A and 15B are partial cross-sectional views of the
lateral electric field type LCD device according to the first
embodiment of FIG. 10 along the lines XVA-XVA and XVB-XVB in FIG.
13, respectively.
[0084] FIG. 16 is a plan view obtained by omitting the pixel
electrode 71 and the common electrode 72 in FIG. 11 for
facilitating the understanding of the understructures thereof.
[0085] FIG. 17 is an enlarged partial plan view obtained by
omitting the pixel electrode 71, the common electrode 72, the black
matrix layer 81, and the contact holes 61 and 62 in FIG. 13 for
facilitating the understanding of the understructures thereof.
[0086] FIG. 18 is a partial cross-sectional view of a lateral
electric field type LCD device according to a second embodiment of
the present invention along the line XVA-XVA in FIG. 13.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0087] Preferred embodiments of the present invention will be
described in detail below while referring to the drawings
attached.
First Embodiment
[0088] FIGS. 11 to 17 show the structure of the active-matrix
substrate of a lateral electric field type LCD device according to
a first embodiment of the present invention. Since all the pixels
of the active-matrix addressing LCD device have the same structure,
the structure of one pixel is shown in these figures.
[0089] In FIGS. 16 and 17, storage capacitor electrodes 73 are
illustrated as a translucent element for facilitating the
understanding of the relationships among common bus lines 52, gate
bus lines 55, storage capacitor electrodes 73, and a common
electrode 72.
[0090] As seen from FIG. 14 and FIGS. 15A and 15B showing the
entire configuration of the device, the LCD device according to the
first embodiment of the invention is configured by coupling and
uniting an active-matrix substrate and an opposite substrate with
each other in such a way that a liquid crystal layer 20 intervenes
between the active-matrix substrate and the opposite substrate.
[0091] As shown in FIGS. 12A, 12B, 12C, 14, 15A and 15B, the
active-matrix substrate of this LCD device comprises a transparent
glass plate 11; and common bus lines 52, gate bus lines 55, drain
bus lines 56, TFTs 45, auxiliary pixel electrodes 70, pixel
electrodes 71, a common electrode 72, and storage capacitor
electrodes 73 formed on or over the inner surface of the glass
plate 11. The common bus lines 52 and the gate bus lines 55, which
are placed directly on the inner surface of the glass plate 11, are
covered with a gate insulating film 57 except for the positions
corresponding to contact holes 62. The drain electrodes 44, the
source electrodes 42, and the semiconductor films 43 of the TFTs
45; the auxiliary pixel electrodes 70; the storage capacitor
electrodes 73; and the drain bus lines 56 are placed on the gate
insulating film 57. Therefore, the common bus lines 52 and the gate
bus lines 55 are electrically insulated from the drain electrodes
44, the source electrodes 42, and the semiconductor films 43, the
auxiliary pixel electrodes 70, the storage capacitor electrodes 73,
and the drain bus lines 56 by the gate insulating film 57. These
structures formed on the glass plate 11 are covered with a
protective insulating film 59 except for the positions
corresponding to the contact holes 61 and 62.
[0092] The pixel electrodes 71 and the common electrode 72 are
placed on the protective insulating film 59. In each of the pixel
regions, the pixel electrode 71 is electrically connected to the
corresponding storage capacitor electrode 73 located right under
the said electrode 71 by way of the corresponding contact hole 61
(which penetrates through the protective insulating film 59).
Furthermore, the pixel electrode 71 is electrically connected to
the corresponding source electrode 42 by way of the corresponding
auxiliary pixel electrode 70 in the pixel region. The common
electrode 72 is electrically connected to the common bus lines 52
located right under the said electrode 72 by way of the
corresponding contact holes 62 (which penetrates through the
protective insulating film 59 and the gate insulating film 57) in
the respective pixel regions. The pixel electrodes 71 and the
common electrode 72 are respectively formed by pattering
transparent conductive metal films, for example, ITO films.
[0093] The surface of the active-matrix substrate having the
above-described structure (i.e., the surface on which the pixel
electrodes 71 and the common electrode 72 are formed) is covered
with an alignment film 31 made of an organic polymer. The surface
of the alignment film 31 has been subjected to a predetermined
aligning treatment for aligning the initial alignment direction of
the liquid crystal molecules existing in the liquid crystal layer
20 to a desired direction.
[0094] On the other hand, the opposite substrate (which may be
termed the color filter substrate) of this LCD device comprises a
transparent glass plate 12; a color filter (not shown) formed by
three color layers 82R, 82G, and 82B of the three primary colors
(i.e., red (R), green (G), and blue (B)), formed on the inner
surface of the glass plate 12 corresponding to the arrangement of
the respective pixel regions; and the black matrix layer 81 for
optical shielding formed on the inner surface of the glass plate
12. In addition, the three color layers 82R, 82G, and 82B are
generically termed the color layer 82.
[0095] The color layer (i.e., the color filter) 82 and the black
matrix layer 81 are covered with an overcoat layer 85 made of an
acrylic resin. Columnar spacers (not shown) are formed on the inner
surface of the overcoat layer 85 to keep the gap between the
active-matrix substrate and the opposite substrate at a constant
value. The inner surface of the overcoat layer 85 is covered with
an alignment film 32 made of an organic polymer. The surface of the
alignment film 32 has been subjected to a predetermined aligning
treatment for aligning the initial alignment direction of the
liquid crystal molecules existing in the liquid crystal layer 20 to
a desired direction.
[0096] The active-matrix substrate and the opposite substrate each
having the above-described structure are superposed on each other
at a predetermined gap in such a way that their surfaces on which
the alignment films 31 and 32 are respectively formed are directed
inwardly and opposed to each other. The liquid crystal layer 20 is
formed in the gap between the active-matrix and opposite
substrates. To confine the liquid crystal material existing in the
liquid crystal layer 20 into the gap between the two substrates,
the outer edges of the two substrates are sealed with a sealing
material (not shown). A pair of polarizer plates (not shown) is
arranged on the outer surfaces of the two substrates,
respectively.
[0097] Next, the structure of the above-described active-matrix
substrate will be explained in more detail below with reference to
FIGS. 11 to 13.
[0098] The gate bus lines 55 extending in parallel to each other at
equal intervals along the lateral or horizontal direction of FIG.
11 and the common bus lines 52 extending in parallel to each other
at equal intervals along the longitudinal or vertical direction of
the same figure define rectangular regions. Each of these
rectangular regions forms a pixel region. These pixel regions
(i.e., the pixels) are arranged in a matrix array as a whole. Each
of the TFTs 45 is located near one of the intersections formed by
the two gate bus lines 55 and the two drain bus lines 56 that
define the pixel region (i.e., at the lower left intersection in
FIG. 11). Similar to the gate bus lines 55, the common bus lines 52
extend along the lateral direction of the same figure in parallel
with the gate bus lines 55. Each of the common bus lines 52 is
located at the opposite side to the TFT 45 (i.e., at the upper end
in FIG. 11) in the pixel region. In other words, it is placed near
one of the two gate bus lines 55 that define the pixel region,
which is located on the distant side from the TFT 45 in the pixel
region, (i.e., the gate bus line 55 located at the upper position
in FIG. 11). Therefore, it may be said that each of the common bus
lines 52 is located near the TFTs 45 existing in the preceding
pixel regions that are upwardly adjacent thereto along the
extension direction of the drain bus lines 56 (i.e., the vertical
direction) to be apart from the TFTs 45.
[0099] The storage capacitor electrode 73 is located at the
opposite side to the TFT 45 (i.e., at the upper end in FIG. 11) in
the pixel region. In other words, the storage capacitor electrode
73 is placed near one of the two gate bus lines 55 that defines the
pixel region, which is located on the distant side from the TFT 45
in the pixel region, (i.e., the gate bus line 55 located at the
upper position in FIG. 11). The storage capacitor electrode 73 is
formed in such a way as to be overlapped with the corresponding
common bus line 52 that is located directly under the said
electrode 73 in such a way that the gate insulating film 57
intervenes between them in the pixel region. The storage capacitor
is constituted by the storage capacitor electrode 73, the
corresponding common line 52, and the gate insulting film 57
intervening between them.
[0100] The above-described structure is the same as that of the
related-art LCD device shown in FIGS. 4 to 10.
[0101] A part (i.e., an upper part in FIG. 11) of the storage
capacitor electrode 73 is projected along the inner surface of the
glass plate 11 toward the side of the preceding pixel regions that
are upwardly adjacent along the extension direction of the drain
bus lines 56 (i.e., the upper side in FIG. 11 and FIGS. 12 A to
12C) compared with the related-art LCD device shown in FIGS. 4 to
10. Therefore, as shown in FIGS. 13, 16 and 17, the projected part
of the storage capacitor electrode 73 is overlapped with the
adjacent gate bus line 55, that is, one of the two gate bus lines
55 that define the pixel region, which is located on the distant
side from the TFT 45 in the pixel region, (i.e., the gate bus line
55 located at the upper position in FIG. 11), in such a way that
the gate insulating film 57 intervenes between them. In this way, a
gate storage structure is formed. In other words, the storage
capacitor electrode 73 is overlapped not only with the
corresponding common bus line 52 located right below the same
electrode 73 to form a storage capacitor (which is the same as the
above-described related-art LCD device shown in FIGS. 4 to 10) but
also with the adjacent gate bus line 55 to form another storage
capacitor (i.e., a gate storage capacitor). Here, the storage
capacitor electrode 73 has an approximately rectangular pattern
(see FIG. 12B).
[0102] As explained later, the projected part of the storage
capacitor electrode 73 is formed to cover the predetermined
non-overlapped area of the adjacent gate bus line 55 that is not
overlapped with the common electrode 72, where the non-overlapped
area of the said gate bus line 55 is located in a part that does
not overlap with the corresponding TFT 45 (in other words, the
removed part of the common electrode 72 near the contact hole 61).
Specifically, as shown in FIGS. 13 and 17, the gate bus line 55
adjacent to the storage capacitor electrode 73 comprises the
non-overlapped area that is not overlapped with the common
electrode 72 (in other words, that is not covered with the common
electrode 72) near the contact hole 61, and the non-overlapped area
is overlapped with the storage capacitor electrode 73. Therefore,
the non-overlapped area of the gate bus line 55, which is not
overlapped with the common electrode 72 and the corresponding TFT
45, is covered with the storage capacitor electrode 73. For this
reason, the electric field generated in the periphery of each gate
bus line 55 can be shielded in the same way as the above-described
related-art LCD device where the whole surface of each gate bus
line 155 is overlapped with the common electrode 172. This
structure makes it possible to shift or displace the position of
the contact hole 61 to a position closer to the edge of the pixel
region in the designing operation, as shown in FIG. 13, while
blocking the electric field generated in the periphery of each gate
bus line 55. This means that the aperture ratio can be improved
easily.
[0103] The auxiliary pixel electrodes 70 are formed in the same
layer as the drain bus lines 56, in other words, they are formed on
the gate insulating film 57. The shape of the auxiliary pixel
electrode 70 corresponds to the shape formed by the combination of
the base of the pixel electrode 171 in the structure of the
related-art LCD device shown in FIGS. 1 to 3 and the central
comb-tooth like part 171a thereof. The bottom end of the auxiliary
pixel electrodes 70 is connected mechanically and electrically to
the source electrode 42 of the TFT 45 and the top end thereof is
connected mechanically and electrically to the storage capacitor
electrode 73 (see FIG. 12B).
[0104] The drain electrode 44, the source electrode 42, and the
semiconductor layer film 43 of the TFT 45 are respectively formed
on the gate insulating film 57 to have such patterns or shapes as
shown in FIG. 12B. The gate electrode (not shown) of the TFT 45 is
formed to be united with the gate bus line 55, in other words, the
gate electrode is a part of the gate bus line 55. The gate
electrode is placed at a position overlapping with the
semiconductor film 43 between the drain electrode 44 and the source
electrode 42. An amorphous silicon film is used as the
semiconductor film 43.
[0105] The pixel electrode 71 and the common electrode 72 provided
for generating liquid crystal driving electric field are
respectively formed to have such patterns or shapes as shown in
FIG. 12C. The pixel electrode 71 and the common electrode 72
comprise comb-tooth like parts (i.e., thin belt-shaped parts
protruding in the pixel region) 71a and 72a that are mated with
each other, respectively. Here, the total number of the comb-tooth
like parts 71a of the pixel electrode 71 is three; on the other
hand, the total number of the comb-tooth like parts 72a of the
common electrode 72 in each pixel region is two.
[0106] The pixel electrodes 71 are provided for the pixel regions
in a one-to-one correspondence. The common electrode 72 is commonly
used for all the pixel regions. Two of the comb-tooth like parts
72a of the common electrode 72 are assigned to each pixel
region.
[0107] The pixel electrode 71 is electrically connected to the
storage capacitor electrode 73 located just below the same pixel
electrode 71 by way of the corresponding contact hole 61
penetrating through the protective insulating film 59 at the base
of the three comb-tooth like parts 71a (which are located on the
opposite side to the source electrode 42). Since the storage
capacitor electrode 73 is electrically connected to the source
electrode 42 of the TFT 45 by way of the auxiliary pixel electrode
70, the pixel electrode 71 is electrically connected to the source
electrode 42 by way of the storage capacitor electrode 73 and the
auxiliary pixel electrode 70.
[0108] The common electrode 72 is electrically connected to the
common bus lines 52 just below the same electrode 72 in the
respective pixel regions by way of the corresponding contact holes
62 penetrating through the gate insulating film 57 and the
protective insulating film 59.
[0109] The common electrode 72 further comprises rectangular
openings or windows 72b formed respectively at the positions
overlapped with the channel regions of the TFTs 45. For this
reason, the whole channel region of the TFT 45 is exposed from the
opening 72b in such a way as not to overlap with the common
electrode 72. This is to avoid the change of the characteristics of
the TFT 45 caused by the back gate effect. The opposite edges of
the common electrode 72 formed by each opening 72b have a smaller
width than the corresponding gate bus line 55, i.e., the gate
electrode.
[0110] As explained above, the common electrode 72 covers not only
the whole surfaces of the drain bus lines 56 extending along the
vertical direction of FIGS. 11 and 13 but also the surfaces of the
gate bus lines 55 extending along the lateral direction of the same
figures except for the openings 72b and the removed parts near the
contact holes 61. To form the removed part, each of the edges 72c
(which extend along the gate bus lines 55) of the common electrode
72 located on the side of the storage capacitor electrode 73 is
bent to have steps. The removed part is covered with the storage
capacitor electrode 73.
[0111] Moreover, similar to the related-art LCD device shown in
FIGS. 4 to 10, the common electrode 72 is formed to cover not only
the areas directly above the gate bus lines 55 but also the gaps
between the gate bus lines 55 and the common bus lines 52 adjacent
thereto (each of the adjacent common bus lines 52 is located in the
subsequent pixel regions that are downwardly adjacent along the
extension direction of the drain bus lines 56, i.e., the vertical
direction), the gaps between the gate bus lines 55 and the
corresponding source electrodes 42, the gaps between the gate bus
lines 55 and the adjacent storage capacitor electrodes 73, and the
peripheral areas of the edges of the source electrodes 42 and the
adjacent storage capacitor electrodes 73. For this reason, the
electric field generated near the gate bus lines 55 is shielded by
the storage capacitor electrodes 73 in the areas that are covered
with the storage capacitor electrodes 73 and by the common
electrode 72 in the areas that are covered with the common
electrode 72. Fringe electric field generated between the edges of
the storage capacitor electrodes 73 and the adjacent gate bus lines
55 is shielded by the common electrode 72.
[0112] The gate bus lines 55 are placed in a lower layer than the
storage capacitor electrodes 73 (i.e., a layer closer to the glass
plate 11), and the common electrode 72 is placed in an upper layer
than the storage capacitor electrodes 73 (i.e., a layer further
from the glass plate 11). Moreover, as shown in FIG. 17, in the
vicinity of each of the non-overlapped areas, the storage capacitor
electrode 73 is overlapped with the corresponding gate bus line 55
in such a way as to exceed across the side edge 55b of the same
gate bus line 55 from a lower position than the side edge 55b to an
upper position than the same. However, the storage capacitor
electrode 73 does not exceed across the side edge 55a of the same
gate bus line 55 opposite to the side edge 55b. The common
electrode 72 is overlapped with the same gate bus line 55 in such a
way as to exceed across the side edge 55a of the same gate bus line
55 from an upper position than the side edge 55a to a lower
position than the same. The common electrode 72 is partially
overlapped with the storage capacitor electrode 73.
[0113] The reference numeral 81 shown in FIGS. 11 and 13 denotes
the black matrix layer formed on the opposite substrate. The black
matrix layer 81 comprises rectangular light-shielding regions
provided for the respective pixel regions. Each of the
light-shielding regions is defined by a rectangular broken line in
FIGS. 11 and 13. Each of the light-shielding regions has a size
that covers the whole TFT 45 and is isolated to have a rectangular
island-like shape at a position right over the TFT 45. In this way,
the occupation area of each light-shielding region of the black
matrix layer 81 is restricted to a minimum necessary for preventing
the entry of light into the TFT 45. The prevention of the entry of
light into (the channel region of) the TFT 45 by the
light-shielding region is to prevent the functions of the TFT 45
from being hindered due to the light.
[0114] With the lateral electric field type LCD device according to
the first embodiment, as shown in FIGS. 13 and 17, the common
electrode 72 comprises not only the openings or windows 72b but
also the removed parts formed near the respective contact holes 61.
The gate bus lines 55 are not covered with the common electrode 72
in the removed parts. For this reason, different from the
related-art LCD device shown in FIGS. 4 to 10, the common electrode
72 does not have a shape covering the whole surfaces of the
respective gate bus lines 55 except for the openings 72b. However,
the non-overlapped areas of the gate bus lines 55 formed near the
contact holes 61 (which are overlapped with the removed parts of
the common electrode 72) are respectively covered with the storage
capacitor electrodes 73 placed in a lower layer than the common
electrode 72. Therefore, similar to the related-art LCD device
shown in FIGS. 4 to 10, the electric filed generated in the
vicinities of the gate bus lines 55 is shielded effectively by the
cooperation of the storage capacitor electrodes 73 and the common
electrode 72. As a result, even if the common electrode 72
comprises the removed parts near the contact holes 61, optical
leakage will not occur near the contact holes 61.
[0115] Moreover, with the above-described related-art LCD device
shown in FIGS. 4 to 10, since the electric filed generated in the
vicinities of the gate bus lines 155 is shielded by the common
electrode 172 alone placed in an upper layer than the gate bus
lines 155, the degree of freedom in designing the pattern and
layout of the constituent elements of the LCD device is limited. On
the other hand, with the LCD device according to the first
embodiment, the parts of the storage capacitor electrodes 73 are
respectively overlapped with the gate bus lines 55 in such a way
that the gate insulating film 57 intervenes between them.
Therefore, the shape of the common electrode 72 is not limited to a
shape covering the whole surfaces of the respective gate bus lines
55 (except for the openings 72b). This shape may be a shape
including non-covering areas that do not cover the respective gate
bus lines 55 in addition to the openings 72b according to the
necessity. In this case, the storage capacitor electrodes 73 needs
to be designed in such a way as to cover the non-overlapped areas
of the gate bus lines 55.
[0116] In this way, with the LCD device according to the first
embodiment, since the storage capacitor electrodes 73 are used for
electric field shielding and light shielding, it is unnecessary
that the common electrode 72 covers the whole surfaces of the
respective gate bus lines 55 except for the openings 72b like the
related-art LCD device shown in FIGS. 4 to 10. As a result, the
degree of freedom in designing the constituent elements is
increased.
[0117] Furthermore, because it is unnecessary that the common
electrode 72 covers the whole surfaces of the respective gate bus
lines 55 except for the openings 72b, the positions of the contact
holes 61 and those of the ends of the pixel electrodes 71 can be
shifted in due order toward the outer edges of the pixel regions.
Therefore, compared with the related-art LCD shown in FIGS. 4 to
10, the aperture ratio can be easily improved, in other words, a
higher aperture ratio can be easily realized.
[0118] Due to the improvement of the aperture ratio, if the amount
of emitted light from the backlight unit is not changed, the
luminance can be increased compared with the related-art LCD
structure shown in FIGS. 4 to 10. If the luminance is not changed,
the electric power consumption can be reduced compared with the
related-art LCD structure shown in FIGS. 4 to 10 due to the same
reason.
[0119] In addition, by properly adjusting the shape and/or the
position of the storage capacitor electrodes 73, a desired storage
capacitance can be ensured easily without lowering the aperture
ratio or while improving the aperture ratio.
[0120] Next, supplemental explanation will be made with respect to
the rectangular openings 72b of the common electrode 72 formed in
the respective pixel regions.
[0121] With the LCD device according to the first embodiment, as
explained above, each of the openings 72b of the common electrode
72 is formed in such a way that the whole channel region (i.e., the
region of the semiconductor film 43 between the source electrode 42
and the drain electrode 44) of the TFT 45 is exposed from the
common electrode 72. At the same time, the rectangular edges (which
extend along the contour of the opening 72b) of the common
electrode 72 formed by the opening 72b, which are located over the
corresponding gate bus line 55, are entirely overlapped with the
same gate bus line 55 at a location where the gate electrode of the
TFT 45 is formed. In other words, the width of (the opposite edges
of) the opening 72b, i.e., the distance between the opposite edges
of the opening 72b along the drain bus lines 56, is sufficiently
smaller than the width of the corresponding gate bus line 55 at a
location where the gate electrode of the TFT 45 is formed (i.e.,
the width of the gate electrode). Accordingly, not only the change
of the characteristics of the TFT 45 due to the back gate effect
can be avoided but also the following advantage is obtained.
[0122] Specifically, even if the alignment direction of the liquid
crystal molecules existing near the opening 72b is changed by the
fringe electric field generated at the edges of the opening 72b,
the incident light emitted from the backlight unit can be blocked
by the gate bus lines 55 made of opaque metal. Therefore, optical
leakage does not occur near the edges of the opening 72b regardless
of the fringe electric field.
[0123] Because of the above-described reason, as shown in the LCD
device of the first embodiment, the occupation area of each
light-shielding region of the black matrix layer 81, which is
formed on the opposite substrate, may be restricted to a minimum
necessary for preventing the entry of external light from the
outside (i.e., the opposite substrate side) into the TFT 145.
Moreover, not only the size of the light-shielding region but also
the Optical Density (OD) value of the light-shielding region may be
restricted. Conventionally, to make sure that the light from the
backlight unit is shielded, the OD value of the light-shielding
region needs to be high. For example, the OD value needs to be
equal to 4.0 or greater, or equal to 3.5 or greater. On the other
hand, with the LCD device of the first embodiment, such the OD
values as described here are not necessary and the OD value may be
lowered. For example, the OD value may be lowered to 3.0 or less,
or to approximately 2.0 or less. For this reason, with the LCD
device of the first embodiment, the requirement for the black
matrix layer 81 is relaxed. As a result, there is an advantage that
the freedom of choice in selecting the material for the black
matrix layer 81 can be expanded and/or the thickness of the black
matrix layer 81 can be reduced. However, to prevent the functions
of the TFT 45 from being hindered due to the irradiation of
external light to the channel region thereof, it is preferred that
the OD value of the lights shielding region is equal to 1.5 or
greater.
[0124] With the related-art LCD device shown in FIGS. 4 to 10, the
width (i.e., the inter-edge distance) of the opening 172b of the
common electrode 172 along the drain bus lines 156 is not
considered.
[0125] Furthermore, with LCD device according to the first
embodiment, as shown in a second embodiment of the invention which
will be explained below, the light-shielding region having a low OD
value may be formed by overlapping at least two of the color layers
that constitute a color filter without using the black matrix layer
81. In this case also, it is preferred that the OD value is equal
to 1.5 or greater. In particular, it is preferred that
shorter-wavelength light such as blue light is shielded as much as
possible. Accordingly, if the light-shielding region is formed by
overlapping at least two of the color layers that constitute a
color filter, it is preferred that at least the color layer for the
red pixel is placed at a position opposite to the channel region of
the TFT 45.
[0126] The LCD device according to the first embodiment having the
above-described configuration may be fabricated as follows, for
example.
[0127] The active-matrix substrate is fabricated in the following
way.
[0128] First, for example, a chromium (Cr) film is formed on the
surface of the glass plate 11 and patterned to have a predetermined
shape. Thus, the common bus lines 52 and the gate bus lines 55
having the shapes shown in FIG. 12A are simultaneously formed on
the surface of the glass plate 11. Next, the gate insulating film
57, which is made of, for example, silicon nitride (SiN.sub.x), is
formed on the whole surface of the glass plate 11 to cover the
common bus lines 52 and the gate bus lines 55.
[0129] Subsequently, the island-shaped semiconductor films 43
(which are usually made of amorphous silicon) for the TFTs 45 are
formed on the gate insulating film 57 in such a way as to be
overlapped with the underlying corresponding gate bus lines 55.
Moreover, for example, a Cr film is formed on the gate insulating
film 157 and patterned, thereby forming simultaneously the drain
bus lines 56, the drain electrodes 44, the source electrodes 42,
the storage capacitor electrodes 73, and the auxiliary pixel
electrodes 70 on the gate insulating film 57. Thereafter, the
protective insulating film 59, which is made of, for example,
SiN.sub.x, is formed on the gate insulating film 57, thereby
covering the drain bus lines 56, the drain electrodes 44, the
source electrodes 42, the storage capacitor electrodes 73, and the
auxiliary pixel electrodes 70.
[0130] Following this, the rectangular contact holes 61 penetrating
through the protective insulating film 59 and the rectangular
contact holes 62 penetrating through the protective insulating film
59 and the gate insulating film 57 are formed. Thereafter, a
transparent conductive film made of ITO is formed on the protective
insulating film 59 and patterned, thereby forming the pixel
electrodes 71 and the common electrode 72 on the protective
insulating film 59. At this time, the pixel electrodes 71 are
electrically connected to the corresponding source electrodes 42 by
way of the corresponding contact holes 61. The common electrode 72
is electrically connected to the respective common bus lines 52 by
way of the corresponding contact holes 62. In this way, the
active-matrix substrate is fabricated.
[0131] The opposite (color filter) substrate is fabricated in the
following way.
[0132] First, the color layers 82R, 82G, and 82B of the three
primary colors for the color filter and the black matrix layer 81
for light shielding are selectively formed on the surface of the
glass plate 12. Thereafter, the overcoat layer 85 is formed on the
whole surface of the glass plate 12 in such a way as to cover the
black matrix layer 81 and the color layers 82R, 82G, and 82B. Then,
the columnar spacers (not shown) are formed on the overcoat layer
85. In this way, the opposite substrate is fabricated.
[0133] Following this, the alignment films 31 and 32, which are
made of polyimide, are respectively formed on the surface of the
active-matrix substrate and the surface of the opposite substrate
fabricated respectively in the above-described manners. The
surfaces of the alignment films 31 and 32 are uniformly subjected
to the predetermined aligning treatments, respectively.
[0134] Thereafter, the active-matrix substrate and the opposite
substrate are superposed on each other to have a predetermined gap
and then, the outer edges of the two substrates are sealed by a
sealing material except for the liquid crystal injection hole.
Subsequently, in a vacuum chamber, a predetermined liquid crystal
material is injected into the space between the two substrates
through the liquid crystal injection hole and thereafter, the
liquid crystal injection hole is closed. After the coupling and
uniting operations of the substrates are completed in this way, the
polarizer plates (not shown) are respectively adhered onto the
outer surfaces of the substrates. As a result, the LCD panel is
completed.
[0135] A predetermined driver LSI (Large-Scale Integrated Circuit)
and a predetermined backlight unit are mounted on the LCD panel
thus fabricated. As a result, the LCD device according to the first
embodiment shown in FIGS. 11 to 17 is completed.
[0136] With the lateral electric field type LCD device according to
the first embodiment, as explained above, the degree of freedom in
designing the constituent elements of the LCD device can be
increased and at the same time, the aperture ratio can be improved
easily compared with the related-art LCD structure shown in FIGS. 4
to 10. Moreover, because of the improvement of the aperture ratio,
if the amount of emitted light from the backlight unit is not
changed, the luminance can be increased compared with the
related-art LCD structure shown in FIGS. 4 to 10. If the luminance
is not changed, the electric power consumption can be reduced
compared with the related-art LCD structure shown in FIGS. 4 to
10.
Second Embodiment
[0137] The structure of a lateral electric field type LCD device
according to a second embodiment of the invention is shown in FIG.
18.
[0138] FIG. 18 is a partial cross-sectional view of the LCD device
according to the second embodiment along the line XVA-XVA in FIG.
13, which is similar to FIG. 15A.
[0139] The structure of the LCD device according to the second
embodiment is the same as that of the LCD device according to the
above-described first embodiment except that the light-shielding
regions are formed on the opposite substrate by overlapping two of
the color layers that constitute a color filter instead of forming
the black matrix layer 81.
[0140] As shown in FIG. 18, at the predetermined positions on the
opposite substrate to be superposed on the TFTs 45, in other words,
at the positions where the light-shielding resions of the black
matrix layer 81 shown in FIG. 13 should be formed, the red color
layer 82R and the green color layer 82G are overlapped with each
other to form light-shielding regions with an increased
light-shielding performance. Such the light-shielding region is not
limited to the combination of the red and green color layers 82R
and 82G; it may be formed by the combination of other color layers.
For example, the light-shielding region may be the combination of
the red and blue color layers 82R and 82B or that of the green and
blue color layers 82G and 82B. This light-shielding region may be
the combination of the three layers, i.e., the red, green, and blue
color layers 82R, 82G, and 82B.
[0141] Such the light-shielding region may be easily formed by the
known method disclosed in the Patent Document 3 or 4, for
example.
[0142] According to the inventors' test, in the case where the
light-shielding region was formed by the combination of the red,
green, and blue color layers 82R, 82G, and 82B, the OD value was
approximately 1.9 when the color specification was defined to
display the chromaticity range of 40% with respect to the NTSC
(National Television System Committee) standard.
[0143] When the color specification was defined to display the
chromaticity range of 60% with respect to the NTSC standard using
the same light-shielding region as above, the OD value was
approximately 2.3. In these two cases, no optical leakage was
observed in the vicinities of the gate bus lines 55 and those of
the TFTs 45.
[0144] Moreover, regarding the effects by incident light from the
external environment, any malfunction such as unusual operation of
the TFTs 45 was not observed even if the LCD device according to
the second embodiment was placed in a severe environment where the
illuminance value on the display screen of the LCD device was able
to reach approximately 100,000 lux.
[0145] As explained above, the LCD device according to the second
embodiment of the invention is the same in structure as the
above-described LCD device according to the first embodiment except
for the structure on the opposite substrate and therefore, it is
apparent that the same advantages as those of the first embodiment
are obtained for the device of the second embodiment.
[0146] In addition, since the LCD device according to the second
embodiment does not comprise the black matrix layer 81, in other
words, this LCD device has a black matrix (BM) less structure,
synergistic advantages are obtained. Specifically, the formation
processes of the black matrix layer 81 can be omitted for realizing
the fabrication cost reduction without degrading conspicuously the
display quality such as display contrast and cross talk. This is
due to the following reason:
[0147] In the case where a LCD device has a BM-less structure, if
the alignment direction of the liquid crystal molecules is affected
by the leaked electric field from the vicinities of the gate bus
lines 55 and/or those of the TFTs 45, usually, there arises
apprehension of contrast lowering and/or cross talk characteristics
degradation. This is because optical leakage occurs due to unwanted
change of the alignment direction of the liquid crystal molecules
in these vicinities. With the structure according to the second
embodiment of the invention, however, the optical leakage due to
unwanted change of the alignment direction of the liquid crystal
molecules in the vicinities of the gate bus lines 55 and the TFTs
45 can be surely avoided. Therefore, even if the LCD device has a
BM-less structure, the fabrication cost can be lowered without
degrading the display quality.
[0148] Furthermore, since the light-shielding region formed by
overlapping the color layers may be restricted to a minimum size at
a position corresponding to the channel region of the TFT 45, no
broadened level difference is formed. Therefore, the problem that
the alignment of the liquid crystal molecules is affected by the
light-shielding region and that the time for the injection process
of the liquid crystal material is prolonged can be avoided. In this
way, aperture ratio improvement and fabrication cost reduction can
be realized simultaneously.
Other Embodiments
[0149] Since the above-described first and second embodiments are
embodied examples of the present invention, it is needless to say
that the present invention is not limited to these embodiments. Any
other modification is applicable to these embodiments.
[0150] For example, the shape or pattern of the storage capacitor
electrode 73 may be optionally changed according to the shape or
pattern of the non-overlapped area of the gate bus line 55 that is
not overlapped with the common electrode 72.
[0151] Moreover, the structure of the LCD device may be optionally
changed except that (i) the drain bus lines 56 are entirely covered
with the common electrode 72, (ii) the gate bus line 55
corresponding to each of the pixel regions is covered with the
common electrode 72 except for the predetermined non-overlapped
area existing in the part that does not overlap with the
corresponding TFT 45, and (iii) the predetermined non-overlapped
area of the gate bus line 55 corresponding to each of the pixel
regions is covered with the storage capacitor electrode 73
corresponding to the adjacent pixel region.
[0152] Although the common electrode 72 is provided for all the
pixel regions in the first and second embodiments, the invention is
not limited to this. A plurality of the common electrodes 72 may be
provided. For example, the common electrodes 72 may be provided for
the pixel regions in a one-to-one correspondence.
[0153] While the preferred forms of the present invention have been
described, it is to be understood that modifications will be
apparent to those skilled in the art without departing from the
spirit of the invention. The scope of the present invention,
therefore, is to be determined solely by the following claims.
* * * * *